The effect of maternal immunization on the antibody response of the offspring to pneumococcal type 19F polysaccharide (PS) was studied. Two- week-old BALB/c mice from mothers immunized with 23-valent pneumococcal vaccine during gestation were given an additional dose of the same vaccine, mouse pneumococcal antiserum, or both. The young mice produced higher titers of antibodies to the 19F PS than the control group that did not receive vaccine or serum after birth. Furthermore, all 11- to 12-week-old monkeys that received an additional dose of 23-valent vaccine, pneumococcal immunoglobulin, or both produced statistically higher titers of IgG antibody to the 19F PS than did controls of various ages. These results suggest that immunization of mothers under certain conditions, such as with an appropriate dose of antigen at a critical period of gestation or postnatal development, could provide young infants with an enhanced antibody response to pneumococcal PS immunogens.